Gents,

I have a 79, f150 4x4 351m/c6. It is new to me. I bought it about two months ago. The previous owner put in a new fuel tank, pump and remain 2 barrel carb (std autolite). When I first bought it, I drove it home about 30 mile with no issues, except for it feeling like the timing may be off. ( my guess). I'm on probably the third tank of gas since buying it. I replaced the fuel filter with and see_through one. Seems to be plenty of fuel.

But now, it's running strangely and I'm not sure where to start. Hoping someone here can help.

Truck starts just fine from cold.
Sporadically, as I'm driving, and holding a constant throttle, the truck will sometimes start to bog down and die....unless I let off of the throttle. After I let the truck coast for a bit or pull over and stop, the truck will run ok again, up to the same speed/ throttle. Then it repeats.

Other times, it won't do that. However, it seems more often than not that it will.

Also, on numerous occasions, once I stop to get gas after driving for a bit, the truck can be extremely difficult to crank and start.

Another symptom is it sounds like it's dieseling.

So, what I need help with is determining if I have a single gel issue..e.g. timing. Or if there may be multiple issues I.e. timing and fuel issues.....but what to start with first.

I'm hoping you guys, based on experience, can point me in the running ghat direction...or sequence of troubleshooting.

I've used the advanced search but haven't found any posted success stories.

Thanks.

 

Hopefully with the new tank the sections of rubber fuel line were replaced. Old rubber lines can collapse and get sucked shut or crack and let air in causing the symptoms you have. I would inspect every inch and replace if not new. There is a short section at the tank, maybe another amidships and another near the pump.

Easy things first.

Also some versions of fuel pump have an integral filter which could be plugged by the old system. Also replace the fuel filter screwed into the carb if not already done.

Check/set timing would be good but I don't think that is your immediate problem.

 

What grade gasoline are you putting in? Does it "diesel" when you shut it off?


First try some higher octane gasoline to fix this issue. My dad had a bone stock 77 F150 with the 351M and we needed to put 89 octane in it. It did not like 87.


Next thing to try would be to reduce the timing a little bit. Your timing may be a few degrees to advanced.

 

So..after some time, I was able to work on the truck. All rubber lines are new proper fuel line/hose. I replaced the fuel pump as a precaution. It is driving a bit better. Now I think the timing may be off. It simply is a noisy, diesel_like engine noise while driving. It does not "diesel" after turning off the engine.

Also, I did replace the fuel filter.

 

Is that diesel like noise detonation? I would sure check that the timing isn't too far advanced. A symptom of that is if it bogs down when starting, like the starter turns hard.
One weird thing I had happen to me, if there is not a heat riser under the carb, I would get carb ice occasionally. The engine would die, after it sat for a while, the ice would melt and it would run good until it iced up again. It's a long shot, but might be worth checking...